Biosensors measure the quantity of a substrate included in sample liquid. The sensors utilize molecular recognition capability of bio material such as germ, enzyme, antibody, DNA, RNA and the like, and uses the bio material as a molecular recognizing element. In other words, when the bio material recognizes an objective substrate, it reacts such that the germ breathes, emits light, consumes oxygen, or causes enzyme reaction. The biosensors utilize those reactions and measure the quantity of the substrate included in the sample liquid. Among the biosensors, enzyme sensors have been promoted to practical use. For instance, an enzyme sensor for glucose, lactic acid, cholesterol, and amino acid is used in medical measurement and food industry. The enzyme sensor reduces an electron carrier with an electron produced by the reaction between the substrate and the enzyme included in the sample liquid, i.e., specimen. A measuring device measures the reduced amount of the electron carrier electrochemically, so that quantative analysis of the specimen is carried out.
Various kinds of biosensors, such as the one discussed above, have been proposed. A conventional biosensor, biosensor Z, will be described hereinafter. FIG. 16(a) shows a perspective exploded view of biosensor Z. FIG. 16(b) shows a structure of an electrode formed at a tip of biosensor Z. A method of measuring a quantity of a substrate in a sample liquid will be described with reference to FIG. 16(b).
First, biosensor Z is inserted into a measuring device. The measuring device applies a given voltage across counter electrode 1103a and measuring electrode 1103b. Then the sample liquid is supplied to inlet 1106b of a sample supplying path. The sample liquid is sucked into the supplying path due to capillary phenomenon, and passes on counter electrode 1103a, which is nearer to inlet 1106b, and arrives at measuring electrode 1103b. Then reagent layer 1105 starts dissolving. At this time, the measuring device detects an electrical change occurring between counter electrode 1103a and measuring electrode 1103b, and starts measuring the quantity. The quantity of the substrate included in the sample liquid is thus measured.
Specifically, oxidoreductase and an electron acceptor retained in the reagent layer dissolve into the sample liquid, and enzyme reaction progresses between the substrate in the liquid. Then the electron acceptor is reduced. After the reaction finishes, the reduced electron acceptor is oxidized electrochemically. A concentration of the substrate can be measured using an oxidation current measured when the acceptor is oxidized.
However, the conventional biosensor Z has some problems to be solved. In particular, when the measuring device detects the electrical change in reagent layer 1105, various factors influence measurement accuracy and sensitivity of the measuring device.
First, an incorrect operation by a user influences them. For instance: (1) After the user supplies the sample liquid to the sample supplying path, the user adds another sample liquid before the measuring device completes the measurement; (2) The user tries to measure the quantity with a biosensor which have been already used; (3) The user supplies the sample liquid to a incorrect place; (4) The user inserts the biosensor into the measuring device in a wrong direction; and (5) When supplying the sample liquid, the user fails to pinpoint an inlet of the sample supplying path, has the sample liquid attach to a surrounding area, and thus has the sample liquid not run into the path. Thus some ways have been desired to avoid those incorrect operations which influence the measurement accuracy. In particular, preventing aged users from the incorrect operations is required.
Second, characteristics of an object to be measured influence them. For instance, when a glucose concentration of human blood is measured with a biosensor, a viscosity of the blood may influence measurement accuracy. Hematocrit, which is generally known as an index of blood viscosity, indicates a volume percentage of erythrocyte included in the blood. Blood in a person who does not suffer from anemia includes 50-60 volume % of water and 40-50 volume % of erythrocyte. If suffering from renal anemia due to chronic renal failure, a person has blood have the volume percentage of hematocrit decrease to less than 15%. Appropriate treatment requires to restrain the influence to hematocrit in the blood for accurate measurement of glucose concentration in the blood of, e.g., a diabetic.
Third, a temperature around the measuring device influences them. Measuring devices available in the market for biosensors have been downsized so that users can carry it with them. Soon after moving into indoors from the outside, a user may try to measure the quantity. In this case, the measurement may start before a temperature in the measuring device becomes stable. A sharp change in temperature influences the oxidation current corresponding to a substrate concentration, and thus may lower the measurement accuracy. A body temperature of the user, upon being transmitted to the measuring device via, e.g., the user's hand, might influence the measurement accuracy.
The present invention thus aims to provide a biosensor being handled easily and having excellent measurement accuracy, a method of measuring quantity using the biosensor, and a measuring device using the biosensor.